Depressive disorders (e.g. Major depressive disorder (MDD)) are the leading cause of disability in the United States when measured as total time lost to disability, affecting more than 18 million people annually in the USA alone. Depressive disorders, the most common of affective illnesses, include a large set of illnesses ranging from seasonal depressive disorder to chronic depression. There are currently no known available biological markers for depression; diagnosis is made by physicians or psychologists based on structured interviews with the patients. Depressive disorders are among only a few major illnesses that remain reliant upon subjective diagnoses. This contributes to under recognition, trivialization and stigmatization of these disabling illnesses.
MDD treatments (e.g. antidepressants and psychotherapy) carry risks of adverse effects, and the economic cost of inappropriate treatment is high. Conversely, the individual, societal, and economic costs of not identifying and treating MDD is even higher, and include suicide, hospitalization, and/or protracted impairment. The costs of misclassification of MDD for research studies are also significant in wasted dollars, time, and incorrect results.